There are numerous fan installations in industry where the fan is required to move air in one direction and then, after a period of time, to move the air in the opposite direction. It is also desirable that a fan be adjusted to move a smaller amount of air without changing the speed of the fan. For example, on the engine of a tracked type of tractor such as a bulldozer or the like it is desirable to have the fan in a neutral or zero pitch position when the engine of the vehicle is being warmed up. When the equipment is being used in the summer, however, it is preferred to have air blown through the radiator and away from the operator but just the reverse is desired in the colder winter months when it is preferred to have the warm air of the radiator blown towards the operator.
A further example is when such equipment is used industry and dirty conditions resulting in radiators being partially plugged or blocked with debris from the environment. It is desirable at such times that the fan be reversed in order to blow out the dust, dirt or other materials from the interstices of the radiator core.
Systems presently available require that the engine fan be stopped so that the blades can be manually adjusted one at a time to set the required pitch, by the operator.
Another example is in the mining industry where, in a mine shaft, fans are used to move air down a shaft and then, after a time, the motors are reversed and the air is exhausted from the mine. These are large diameter fans and require large motors of substantial horsepower. The stopping, starting and reversing of these motors is time consuming and expensive.
In large agricultural operations it is required to keep the air in buildings at a constant temperature during changing outside air temperature levels. A variable speed reversing fan which is temperature controlled is expensive when compared to a constant speed fan with variable pitch blades.
There are fans on the market which are reversible but they do not move air in both directions with equal efficiency. There are also some designs which disclose adjustable blades but they are limited in the number of blades and have inherent friction and lubrication problems associated with the inner components of the fan assemblies. Such fans have never come into production due to these problems. There is also the problem of the physical size associated with the available adjustable blade fans which prohibits their use in many vehicular applications. For example, one existing fan has fan blades that include an airfoil configuration similar to an airplane wing so that the fan blades have differential blade surfaces on either side of the blade. Hence, the fan does not provide equal fore and aft thrust.
Another fan uses an airscrew shape, in which the fan blade is twisted along its length. However, this fan blade needs a relatively large amount of space in which to turn to be orientable from forward to rearward operation.